Artificial lighting devices are used to provide light at a desired intensity and location, and can be fixed, such as street lights, or mobile, such as hand-held torches. Artificial lights are used to illuminate dark areas, such as interiors of buildings or outdoor spaces at night. Illuminating dark areas can be used, for example, to facilitate navigation, improve security and safety, extend working and production hours, and increase leisure time. Examples of artificial lights include street lights, torches, floodlights, fluorescent light globes, and filament light globes.
In some applications, artificial lights are utilised to provide illumination of a predetermined area, such as a street or path. Controlling the intensity and/or the direction of light from an artificial lighting device can also be utilised to create atmosphere or ambience, such as in a restaurant. Another application of artificial lighting devices is to focus light in a predetermined manner to guide and control the movement of people, vessels, and vehicles. Such lighting devices include, for example, beacons, warning lights, lighthouses, headlights, tail-lights, and traffic signal lanterns.
Traditionally, signal lanterns have used incandescent filament lamps or quartz halogen lamps as a source of artificial light. The lamp is fitted at the focus of a parabolic reflector and the front of the reflector is fitted with a coloured lens that determines the colour of the signal. More recently, signal lanterns have been implemented using light emitting diodes (LEDs) as a light source. The LED lanterns, when compared with lanterns utilising incandescent filament lamps, have the advantage of lower power consumption and longer life.
Current lanterns use light sources that suffer a reduction in light output as those light sources age. This loss of light, which is often called lumen depreciation, causes designers to make lanterns that produce excessive light and consume excessive power in the early part of the lanterns' lives. The excess light can be so great as to be harmful and the extra power is just wasted. The production of excessive light and consumption of excessive power also reduces the lifetime of the LED.
For some kinds of LED, especially those used in red and yellow traffic light signals, the light output depends strongly on the operating temperature of the LED. The operating temperature is further affected by the local ambient temperature and by heating due to solar radiation. This again leads designers to compensate by applying extra power to the LEDs. Applying extra power to the LEDs exacerbates the power consumption and lumen depreciation problems. The combined effect is large and makes the design of red lanterns particularly problematic. LEDs work most efficiently when cold and least efficiently when the LEDs are hot, whereas the required light output is greatest during the day and least during the night.
In order to reduce the light output of an LED signal lantern for use at night, it may be necessary to apply less than a standard voltage to the LED signal lantern. However, typical LED signal lanterns have poor power factor and are difficult to operate with less than the standard voltage.
For the purpose of dimming the light output of a signal lantern, one approach is to reduce an applied voltage by reducing the amplitude of the applied alternating current (A.C.) mains voltage. Alternatively, a second approach uses a method known as “phase dimming” to dim a signal lantern, by removing part of the applied mains wave form through the use of a control element, such as a TRIAC. Both forms of dimming of a signal lantern can give the same applied root mean square (RMS) voltage. However, LED signal lanterns commonly produce different amounts of output light when different methods of dimming are utilised. This is in contrast to traditional incandescent lanterns, which do not behave in this manner and generally produce the same amount of output light, irrespective of the type of dimming method that is utilised.
An additional problem occurs when it is desired to determine the number of lanterns connected to a control system by measuring the total power consumed. This is readily determined when using incandescent lanterns, as the incandescent lanterns behave in a consistent manner. Since the relationship between an applied voltage and the consumed power for a LED signal lantern is commonly not the same as the voltage power relationship of an incandescent lantern and, more seriously, the relationship is also dependent on the applied voltage waveform, it is difficult to use power consumption as a means for assessing the number of LED lanterns connected to the control system.
Thus, a need exists to provide an improved method and system for controlling power supplied to electric lighting devices.